WASHINGTON (CNN) -- A U.S. destroyer has entered Somali territorial waters in pursuit of a Japanese-owned ship loaded with benzene that was hijacked by pirates over the weekend, military officials said Monday.

The destroyer USS Porter, shown in 2006, sank pirate skiffs tied to the stern of the hijacked vessel.

The guided-missile destroyer USS Arleigh Burke entered Somali waters with the permission of the troubled transitional government in Mogadishu, U.S. officials said. In recent years, warships have stayed outside the 12-mile limit when chasing pirates.

The ongoing operation was confirmed to CNN by two military officials familiar with the details.

Gunmen aboard two skiffs hijacked the Panamanian-flagged Golden Mori off the Socotra archipelago, near the Horn of Africa, said Andrew Mwangura, a spokesman for Kenya's Seafarers' Assistance Programme.

The Golden Mori radioed for help Sunday night. The Burke's sister ship, the USS Porter, opened fire and sank the pirate skiffs tied to its stern before the Burke took over shadowing the hijacked vessel.

When the shots were fired, it was not known the ship was filled with highly flammable benzene. U.S. military officials indicate there is a great deal of concern about the cargo because it is so sensitive.

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Benzene, which U.S. authorities have declared a known human carcinogen, is used as a solvent and to make plastics and synthetic fabrics.

U.S. and NATO warships have been patrolling off the Horn of Africa for several years in an effort to crack down on piracy off Somalia, where a U.N.-backed transitional government is struggling to restore order after 15 years of near-anarchy.

In June, the destroyer USS Carter Hall fired warning shots in an attempt to stop a hijacked Danish cargo ship off Somalia, but the American vessel had to turn away when the pirated ship entered Somali waters.

In May, a U.S. Navy advisory warned merchant ships to stay at least 200 miles off the Somali coast. But the U.S. Maritime Administration said pirates sometimes issue false distress calls to lure ships closer to shore.

The pirates are often armed with automatic rifles and shoulder-fired rockets, according to a recent warning from the agency.

"To date, vessels that increase speed and take evasive maneuvers avoid boarding, while those that slow down are boarded, taken to the Somali coastline and released after successful ransom payment, often after protracted negotiations of as much as 11 weeks," it advised.

The agency issued a new warning to sailors in the Gulf of Aden, between Somalia and Yemen, after Sunday's hijacking was reported.E-mail to a friend